I have been running with 85ah lesuire battery which when I first used it completed Boo Boo's run with out losing a bar on the C5 battery indicator - recently ,however, it's not been performing too well and it is under a year old. For some strange reason I decided to read the instruction's on my charger an noted the comment 'not suitable as a caravan battery charger' of which this battery is. I was always a fan of slow and gentle charges to look after the battery - however I don't my super safe charger is giving enough amps to fully charge it - found this on the internet :
A new, fully charged battery produces 2.1 volts per cell or 12.6 volts for a 12 volt battery. You must understand what voltage and amperage mean. Voltage is a measure of electrical pressure. Amperage is the amount of current passing through a conductor. As a battery discharges - goes dead - the electrical resistance increases especially with todays maintenance-free batteries. A 6 amp charger probably will not fully charge your battery since the internal resistance of the dead battery is so high. You should slow charge the battery but I recommend a charger with an output of 10-12 amps, see if you can borrow one.
Going to up the amps and see if it helps - ps so much for my ONC in electronic engineering !!!